Background: Previous work has shown that iron-based Fischer-Tropsch synthesis (FTS)
catalysts can be modified to give a better performance in the synthesis reaction by treatment with microwave
radiation. Iron-manganese systems have not previously been studied for this effect but are of
interest because of their claimed special FTS performance.
Methods: Iron-manganese catalysts were prepared and modified by exposure to microwave radiation.
Their catalytic performance in FTS was measured and the surface properties were monitored using the
temperature programmed surface reaction (TPSR) of prechemisorbed CO with hydrogen.
Results: Only relatively slight changes in the bulk structure of the solids could be seen in microwaved
samples, except at higher power levels and longer durations, when some growth in the size of the iron
particles was seen. The reduction characteristics of catalysts were also relatively unaffected by microwave
radiation. Microwaved catalysts tended to be slightly less active in the FTS reaction but were
more stable, with less deactivation being seen. The surface of the catalyst was modified by the microwave
radiation, leading to more strongly and more dissociatively adsorbed CO as detected by TPSR.
Conclusion: Microwave radiation improves the performance of unsupported iron-manganese
catalysts, but the effects seen, although significant, are not as dramatic as those recorded for iron
catalysts. A contributory factor may be the absence of potassium which has been implicated in
microwave-induced surface modifications of potassium-promoted iron, but the intrinsic microwave
absorption properties of manganese-containing samples may also be responsible. The radiation affects
surface rather than bulk characteristics.